Piezoelectric ceramic materials, based on lead-ziconate-titanate (PZT), comprising valence-compensated complexes containing Ag

ABSTRACT

The characteristics of piezoceramic multilayer actuators based on lead-zirconate-titanate are determined to a great extent by the compatibility of PZT ceramics having a low sintering temperature with the AgPd internal metallisation during cofiring. It is important to take into consideration that Ag ions in PZT modifications have a high diffusivity at high temperatures (&gt;800° C.) and in addition act as acceptor doping when integrated into the PZT system. The reduction of the fraction of the precious metal palladium, which prevents diffusion, is limited, as silver increasingly diffuses into the piezoceramic as the silver fraction in the internal electrodes increases. According to the invention, Ag +  ions are used to form valence-compensated compositions of the PZT system. A higher level of deformation is maintained, i.e. the acceptor-donor effect in the system is very similar to that of the PZT system modified conventionally without internal electrodes.

This is a §371 of PCT/DE02/03656 filed Sep. 26, 2002, which claimspriority from German 101 48 333.3 filed Sep. 29, 2001 and German 102 31471.3 filed Jul. 12, 2002 all of which are hereby incorporated byreference in their entireties.

The invention relates to the modification of piezoelectric ceramicmaterials based on lead zirconate titanate (PZT).

The properties of piezoceramic multi-layer actuators based on leadzirconate titanate are determined to a great extent by the compatibilityof low-sintering PZT-ceramic with the AgPd internal metalization duringco-firing.

It should be noted here that Ag ions in PZT modifications have highdiffusivity at high temperatures (>800° C.) and also act as an acceptordopant when incorporated into the PZT system. A non-, or only partly,compensated acceptor substitution by donor ions such as Nb⁵⁺ can lead todrastic changes in the ferroelectric properties and thus also in thedeformation behavior of actuator materials. This effect must be takeninto account when selecting the AgPd alloy for the internal electrodes.There is a limit to how far the content of the noble metal palladium canbe reduced, as an increase in the silver content of the internalelectrodes leads to increased diffusion of the silver in thepiezoceramic.

The use of the alloy AgPd 70/30 for the internal metalization fulfillsthe premises for compatibility with the piezoceramic in the cofiringprocess. A Pd content of ≧30% has the effect of preventing the diffusionof silver.

Ag-additives to originally valence-compensated PZT modifications act asacceptor dopants, which influence the polarity behavior. This alsoresults in a reduction in deformation in the vicinity of an electricfield.

Accordingly, the deformation characteristic is not stabilized byanticipating the introduction of Ag⁺ ions when sintering piezoceramiclaminates with AgPd internal metalization by adding Ag when formulatingthe composition of conventionally modified PZT compositions.

The object of the invention is to prevent diffusion of Ag in the ceramicmaterial as far as possible in spite of a reduced Pd content of theelectrode material.

The object is achieved with the aid of the present invention.

The invention consists in that in the modifications of piezoelectricceramic materials based on lead zirconate titanate asvalence-compensated compositions with n complex compounds in themulti-component systems with the general formulaPbTiO₃—PbZrO₃-Σ_(n)AM′_(α)M″_(β)O₃ with heterovalent ions, at least oneion M′ is represented by Ag⁺ and M″ are pentavalent cations of Sb, Ta,Nb, wherein n≦3, α=¼, β=¾, the content of the respective complexcompounds being ≦20 mol % and A≡A²⁺, populated by ions of the elementsPb, Ba, Ca, or Sr.

The multi-component systems with the general formulaPbTiO₃—PbZrO₃-Σ_(n)A′_(α)A″_(β)BO₃ with heterovalent ions are modifiedso that at least one ion A′ is represented by Ag⁺ and A″ are trivalentcations of Bi, La, Nd, wherein n≦3, α=β=½, the content of the respectivecomplex compounds being ≦20 mol % and B≡B⁴⁺, populated by ions of theelements Ti or Zr.

In the modified system, the hypothetical complex Sr(Ag_(0.25)Nb_(0.75))O₃ in a PZT-SAN(1−y)Pb(Zr_(x)Ti_(1-x))O₃-ySr(Ag_(0.25)Nb_(0.75))O₃ system, for example,may be considered as a coupled substitution of Ag⁺ acceptor ions andNb⁵⁺ donor ions.

The composition0.98Pb(Zr_(0.52)Ti_(0.48))O₃-0.02Sr(Ag_(0.25)Nb_(0.75))O₃ of the PZT-SANsystem with an Ag content of 0.178 wt. % was compared with the modelsubstance with the non-stoichiometric composition0.98Pb(Zr_(0.52)Ti_(0.48))O₃-0.02Sr(Ag_(0.00)Nb_(0.75))O₃ without Ag⁺ions for acceptor-donor valence compensation. The properties show thatit is not possible to achieve valence compensation by removing theacceptor deficit via the diffusion of Ag from the internal electrodes.The deformation of the non-stoichiometric composition is substantiallyless than that of the stoichiometric composition.

The invention is explained in more detail with the aid of an embodiment.The polarization and deformation behavior of laminates with and withoutinternal electrodes was investigated. The interaction of the modifiedPZT ceramics with different AgPd alloys (AgPd 70/30 and AgPd 85/15)during cofiring of the laminates is assessed and a solution for thereduction of the Pd content <30 is indicated.

Knowledge of effective x- and y-concentrations arises from thedevelopment of the PZT system: The properties relevant for theapplication of the modified PZT ceramics for sensors and actuators canbe optimized in a narrow concentration range of x=0.46 to 0.49 fory=0.02. Accordingly, in an example, a PZT-SAN composition series 0.98Pb(Zr_(x)Ti_(1-x))O₃-0.02 Sr(Ag_(0.25)Nb_(0.75))O₃ for x=0.51 to 0.55was formulated in steps from Δx=0.005 and synthesized by the mixed oxidemethod.

When synthesizing (sintering) the PZT-SAN ceramics, an increased actionof Ag⁺ ions as acceptor dopants with coupled substitution of Nb⁵⁺ donorions was already

The characteristic functional properties (piezoelectric and dielectriccoefficients) of the compositions0.98Pb(Zr_(0.52)Ti_(0.48))O₃-0.02Sr(Ag_(0.25)Nb_(0.75))O3 and0.98Pb(Zr_(0.53)Ti_(0.47))O₃-0.02Sr(Ag_(0.25)Nb_(0.75))O₃ from thisseries of compositions are given in the following table as examples.

x = 0.52 x = 0.53 Curie temperature T_(c), ° C. 350 348 Coercive fieldstrength E_(c), 10⁶ V/m 1.27 1.11 Permittivity ε₃₃ ^(T)/ε₀ 1959 2077Dielectric loss factor tan δx 10⁻⁴ 129 145 Planar electromechanicalcoupling k_(p) 0.62 0.66 factor Piezo modulus d₃₃, pC/m 423 498Re-orientation S₃, 0/00 2.09 2.16 deformation (E = 3 · 10⁶ V/m)

Dielectric and electromechanical properties of PZT-SAN ceramics

When forming valence-compensated compositions of the PZT-SAN system, ahigh level of deformation is obtained upon polarization.

The influence of the diffusion of silver during cofiring of thelaminates with internal electrodes of differing Pd contents is lesspronounced. The values of the coercive field strength and deformationfor the composition0.98Pb(Zr_(0.52)Ti_(0.48))O₃-0.02Sr(Ag_(0.25)Nb_(0.75))O₃ of the PZT-SANsystem for example are virtually the same for both internalmetalizations, as can be seen from FIGS. 1 and 2.

FIG. 1 shows the polarization behavior and

FIG. 2 shows the deformation behavior of sintered laminates of0.98Pb(Zr_(0.52)Ti_(0.48))O₃-0.02Sr(Ag_(0.25)Nb_(0.75))O₃ ceramics withand without AgPd internal electrodes.

FIG. 2 shows the deformation behavior of sintered laminates of0.98Pb(Zr_(0.52)Ti_(0.48))O₃-0.02Sr(Ag_(0.25)Nb_(0.75))O₃ ceramics withand without AgPd internal electrodes.

Reducing the diffusion of silver during cofiring of laminates withinternal electrodes makes it possible to use AgPd alloys with reduced Pdcontent. The values of coercive field strength and deformation for thecomposition of 0.98Pb(Zr_(0.52)Ti_(0.48))O₃-0.02Sr(Ag_(0.25)Nb_(0.75))O₃of the PZT-SAN system for internal metalization with AgPd 70/30 and AgPd85/15 are virtually the same. When using Ag⁺ ions to formvalence-compensated compositions of the PZT system, a high level ofdeformation is still obtained, i.e. the acceptor-donor action of thesystem is very similar to that of the conventionally modified PZT systemwithout internal electrodes, as can be seen from FIG. 2.

1. A modification of a piezoelectric ceramic material based on lead zirconate titanate of the formula (1−y)Pb(Zr_(x)Ti_(1-x))O₃-yA²⁺(Ag_(α)B_(β) ⁵⁺)O₃, wherein A²⁺ is an ion of an element selected from the group consisting of Pb, Ba, Ca and Sr and B⁵⁺ is an ion selected from the group consisting of an element Sb, Ta and Nb and y=0.001 to 0.10, x=0.45 to 0.55 and β=¼ and β=¾.
 2. A modification of a piezoelectric ceramic material based on lead zirconate titanate as a valence-compensated composition with an n complex compound in the multi-component system having the general formula PbTiO₃—PbZrO₃-ΣAM′_(α)M″_(β)O₃ with heterovalent ions, wherein M′ is Ag⁺ and M″ is a pentavalent cation selected from the group consisting of Sb, Ta, and Nb, wherein n≦3, α=¼, β=¾, wherein the content of the respective complex compounds is ≦20 mol. %, and A≡A²⁺, and is an ion of an element selected from the group consisting of Pb, Ba, Ca, and Sr.
 3. A modification of a piezoelectric ceramic material based on lead zirconate titanate as a valence-compensated composition with an n complex compound in the multi-component systems with the general formula PbTiO₃—PbZrO₃-Σ_(n)A′_(α)A″_(β)BO₃ with heterovalent ions, wherein A′ is Ag⁺ and A″ is a trivalent cation selected from the group consisting of Bi, La and Nd, wherein n≦3, α=β=½, wherein the content of the respective complex compounds is ≦20 mol. % and B≡B⁴⁺, and is an ion of an element selected from the group consisting of Ti and Zr.
 4. A modification of a piezoelectric ceramic material based on lead zirconate titanate as a valence-compensated composition with an n complex compound in the multi-component system having the general formula PbTiO₃—PbZrO₃-Σ_(n)AM′_(α)M″_(β)O₃ with heterovalent ions, wherein M′ is Ag⁺ and M″ is a pentavalent cation selected from the group consisting of Sb, Ta, and Nb, wherein n≦3, α=¼, α=¾, wherein the content of the respective complex compounds is ≦20 mol. %, and A≡A²⁺ and is an ion of an element selected from the group consisting of Pb, Ba, Ca, and Sr, and wherein the Pb ion in the base system PbTiO₃—PbZrO₃(PZT) is substituted partially, by up to 10 mol %, with at least one divalent ion selected from the group consisting of Ba, Ca and Sr.
 5. A modification of a piezoelectric ceramic material based on lead zirconate titanate as a valence-compensated composition with an n complex compound in the multi-component systems with the general formula PbTiO₃—PbZrO₃-Σ_(n)A′_(αA″) _(β)BO₃ with heterovalent ions, wherein A′ is Ag+ and A″ is a trivalent cation selected from the group consisting of Bi, La and Nd, wherein n≦3, α=β=½, wherein the content of the respective complex compounds is ≦20 mol %, and B≡B⁴⁺, and is an ion of an element selected from the group consisting of Ti and Zr wherein the Pb ion in the base system PbTiO₃—PbZrO₃(PZT) is substituted partially, by up to 10 mol %, with a divalent ion of an element selected from the group consisting of Ba, Ca and Sr. 